I will admit that I a fairly new to the pond world but from what I understand, if you don't have a hole in the ice, there will be no oxygen exchange. Gases will build up under the ice and fish could die from lack of oxygen.

Must be getting awfully cold for your water to freeze with both heaters working. If I remember correctly, one of the heaters was fairly large. Are you sure that you didn't lose power to the heaters?
Yes, breaking a hole in the ice is dangerous. The shock wave that is created in the water when you are breaking the ice can be deadly to the fish.
Do you have any idea how thick the ice is?
Sorry that I didn't catch this when it was first posted.